The methods described herein were made by employee(s) under contract with the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
Aluminum alloys are used in a variety of applications. For example, certain aluminum alloys are extensively employed in lightweight structures, such as are present in subsonic aircraft systems, because of their high strength to weight ratio, low cost and good formability. These high strength alloys, designated in the art as 7000 series aluminum alloys, contain as their principal components a major amount of aluminum and minor amounts of zinc.
Friction stir welding is utilized to join engineered components (hereinafter referred to as xe2x80x9ccomponent [singular] or components [plural]xe2x80x9d) to form assemblies that may be used in the manufacture of aerospace, aeronautical, military and commercial applications. As illustrated in FIG. 1, friction stir welding involves inserting a rotating friction stir welding probe 10 (hereinafter referred to as xe2x80x9cprobexe2x80x9d) into a multiplicity of faces of faying components, while urging the components together. The rotation of the probe 10 between the faying faces of the components creates friction that generates sufficient heat energy to plasticize the components"" material in the weld zone 14. A weld joint forms 13, joining the components together in a unitary assembly 15, as the plasticized regions of the components flow together and solidify in the weld zone. Alternatively, a plasticized region between the components can be formed without utilizing a rotating tool by moving one of the components relative to the other while concurrently urging the components together.
Although the development of friction stir welding technology to make complex welds is proceeding at an extremely rapid pace, understanding of the microstructural transformations that occur during the friction stir welding process and of the post weld mechanical properties has been relatively slow. Where 7X50 (wherein xe2x80x9cXxe2x80x9d can represent any number from 0-9, see block 31 of FIG. 5) series aluminum (Al) alloy components are friction stir welded, solution heat treating the components is necessary to improve corrosion resistance (including stress corrosion cracking [SCC] resistance), ductility as well as tensile and yield strengths. Solution heat treatment for the purposes herein is defined as heating an alloy to a suitable temperature and holding the alloy at that temperature long enough to cause one or more constituents to enter into a solid solution. Post-friction stir welded heat treatment of 7X50 aluminum alloy components in accordance with SAE AMS-H 6088, a standard heat treat specification with temperature schedules commonly known in the art, results in large columnar grains in the weld metal of a friction stir welded joint. Further, elongation is below 2% and standard 3-point bend specimens fracture in a brittle manner. A fracture of this nature, as is known in the art, is considered to be unacceptable. The standard practice uses specific temperature schedules in an effort to mitigate SCC problems on 7X50 series aluminum alloys that have not been welded or otherwise thermally processed. Most research is currently attempting to use low temperature heat treatments for shorter time periods to eliminate SCC concerns as well as elimination of a heat treatment step altogether. Further, an advantage of the low temperature heat treatment has a less likelihood to distort a component. The term low temperature as used herein is defined as less than about 400 degrees F. Conversely, the term high temperature for the purposes herein is defined as greater than about 410 degrees F.
Various types of low temperature schedules in efforts to mitigate material degradation after a friction stir weld process have been attempted. However, material degradation such as SCC may still exist. The present method uses high temperature heat treatment in contrast to the current research trend.
Thus, there is a need for improved methods of heat treating friction stir welding materials. Such manufacturing methods should be cost effective, as well as minimizing the degradation of the material properties.